This invention is drawn to the field of underwater acoustics, and more particularly, to a very high resolution scanning within-pulse quadrature sampled phase shift beamformer for a circular or cyclindrical soundhead, and still more particularly, to such a beamformer that corrects for fill time and focuses at all nearfield ranges of interest.
U.S. Pat. No. 4,233,678, issued Nov. 11, 1980 to Brady, incorporated herein by reference, provides a within-pulse quadrature sampled scanning phase shift beamformer for beamforming a soundhead having a circular or cylindrical array of hydrophone elements. Under control of a clocking signal of constant frequency, an analog multiplexer scans the array and sequentially shifts inphase (I.sub.i) and quadrature phase (Q.sub.i) samples from successive hydrophones of the soundhead into a serial-in/parallel-out charge transfer device operated as a transversal filter. The parallel outputs are resistively phase shifted and combined on a pair of differential buses for forming the Inphase (I) and Quadrature phase (Q) beam component signals corresponding to a particular azimuthal beam direction (k) as represented by the equations: ##EQU1## n is the number of azimuthally ordered hydrophones used to form beam k, W.sub.i represents the shading factor used for side lobe suppression, and cos .theta..sub.i and sin .theta..sub.i represent the phase shift operator used for steering the array. The magnitude of the beam for a particular azimuthal beam direction (k) is calculated by an approximation to the square root of I.sub.k.sup.2 +Q.sub.k.sup.2. The output of Brady is thus a sequence of consecutive azimuth-ordered beams which occur at the rate at which the quadrature sampled outputs of the hydrophones are sequentially supplied to the charge transfer device from the analog multiplexer, which rate is selected to ensure that all beams are formed within a ping length.
Such a quadrature sampled phase shift beamformer, however, is performance limited because the degree of resolution that can be obtained is restricted by an effect called the array fill time. Since the phase shift operator modifies the relative phases of the quadrature sampled outputs in a manner where coherent combination is accomplished without regard to the signal time of arrival, the soundhead is not full whenever the leading edge of the actual signal is entering the array or the trailing edge is clearing the array, leading to such effects as lower sensitivity, wider beamwidth and pulse stretching.
Another factor that materially impacts the degree of the resolution obtainable is the capability of the beamformer to focus, that is, to accommodate the spherical wavefront arising from within the nearfield of the soundhead rather than approximating it as a plane wave. The plane wave approximation is accurate for signals arising from within the farfield of the soundhead. The nearfield/farfield transition region is commonly assumed to be located about d.sup.2 /.lambda. away from the soundhead, where d is the characteristic dimension of the beamforming aperture and .lambda. is the sonar wavelength. From the beamformer standpoint, however, it is clear that the nearfield/farfield boundary (R') decreases in range as resolution improves. Therefore, the performance of very high resolution systems which are sure to operate with targets in the very nearfield is resolution limited by the capability of the beamformer to focus.